$\dfrac{8}{2} - \dfrac{1}{10} = {?}$
$ = {\dfrac{8 \times 5}{2 \times 5}} - {\dfrac{1 \times 1}{10 \times 1}} $ $ = {\dfrac{40}{10}} - {\dfrac{1}{10}} $ $ = \dfrac{{40} - {1}}{10} $ $ = \dfrac{39}{10}$